[FIX] hr_holidays: Ensure the name is mandatory in public holidays

The name field was previously optional when adding a new instance
from the tree view, which in the absence of a name, 'false' is indicated
in the calendar view. The field is now set as mandatory.

closes odoo/odoo#132772

Task: #3469914
Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>